Some additional assumptions are needed. The first one is that
oil cannot be taken up through pores which are still expelling
water vapour, since it would be displaced by the vapour flow. Second,
the total amount of pores is formed at the maximum evaporation
rate, at which point all pores are active. showed that there is a linear relationship between the pressure
gradient over a membrane and the percentage of active pores.
Additionally, from Darcy’s law we know that the flux through a
porous medium is proportional to the pressure gradient. Following
this we can relate the water evaporation rate and the amount of
active pores using: